The sense of hearing in human beings involves the use of hair cells in the cochlea that convert or transduce audio signals into auditory nerve impulses. Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded. These sound pathways may be impeded, for example, by damage to the auditory ossicles. Conductive hearing loss may often be helped by the use of conventional hearing aids that amplify sound so that audio signals reach the cochlea and the hair cells. Some types of conductive hearing loss may also be treated by surgical procedures.
Sensorineural hearing loss, on the other hand, is caused by the absence or destruction of the hair cells in the cochlea which are needed to transduce acoustic signals into auditory nerve impulses. People who suffer from sensorineural hearing loss may be unable to derive significant benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is. This is because the mechanism for transducing sound energy into auditory nerve impulses has been damaged. Thus, in the absence of properly functioning hair cells, auditory nerve impulses cannot be generated directly from sounds.
To overcome sensorineural hearing loss, numerous cochlear implant systems have been developed. Cochlear implant systems bypass the hair cells in the cochlea by presenting electrical stimulation to the auditory nerve fibers by way of one or more channels formed by an array of electrodes implanted in the cochlea. Stimulation of the auditory nerve fibers leads to the perception of sound in the brain and at least partial restoration of hearing function.
Unfortunately, conventional cochlear implant systems cannot present a full spectrum of audible sound to the patient. For example, there are often sounds that have pitches lower than those which can be conventionally generated by applying electrical stimulation to one or more electrodes disposed within the cochlea of a patient. This is especially the case when ossification, malformations within the cochlea, and/or other anatomical anomalies prevent full insertion and/or function of an electrode lead within the cochlea.
Intraneural stimulation has been proposed as an alternative to intracochlear stimulation that may facilitate better representation of relatively low frequencies and finer spectral resolution throughout the entire hearing spectrum. In intraneural stimulation, an electrode array is inserted into the auditory nerve (e.g., at the base of the cochlea). Individual electrode contacts disposed on the electrode array may then be used to directly stimulate individual nerve fibers included in the auditory nerve, thereby allowing an intraneural stimulation system to convey a full spectrum of audible sound to the patient.
Unfortunately, however, the nerve fibers within the auditory nerve are arranged in a helical fashion such that a transverse section of the of the auditory nerve will not represent a monotonic organization of pitch. Hence, an electrode array that has been inserted into the auditory nerve would result in non-monotonic pitch percepts when its electrode contacts are stimulated sequentially from one end of the array to the other. This makes it difficult to fit an intraneural stimulation system to a patient (i.e., determine which electrode contacts correspond to which frequencies). The fitting difficulty is exacerbated if the patient is incapable of providing subjective feedback during a fitting session (e.g., if the patient is a child).